1. Field of the Invention
The invention generally relates to the field of optical fibers, and more particularly, to making optical fiber preforms from which optical fibers are drawn.
2. Related Art
The widespread use of optical fibers (lightguides) in telecommunications, medicine, and metrology can, in general, be directly related to the advent of low-loss glass fibers. Though some loss is inherent, low loss fibers result from reducing the loss mechanisms incorporated in the fiber during manufacture. These mechanisms include, among others, impurities that act as light absorbers, and geometrical distortions that lead to scattering of light inside the fiber.
Additionally, widespread deployment of low loss optical fibers has generally required that fibers possess the material strength to withstand placement in harsh environments. This has been facilitated by reducing structural faults, such as bubbles or chemical impurities, that can cause significant mechanical stress and weakening of the fiber and/or can cause added loss. Typically, the loss mechanisms and structural faults in an optical fiber drawn from a glass preform result from these imperfections existing in the glass preform. Thus, to manufacture high strength, low loss glass fibers efficiently, techniques must be employed that reduce the loss mechanisms and structural faults present in the glass preform.
When loss mechanisms and structural faults result from preform surface imperfections, they can be substantially eliminated by removing surface material comprising the imperfection (this removal process being referred to as an etching process). Etching techniques, such as mechanical milling, chemical etching and plasma etching, are available that can be applied to glass preforms.
Conventional chemical etching is relatively slow and is typically not a clean process. Though some imperfections are removed by chemical reaction, different imperfections can be incorporated as a byproduct of the etching reaction. In addition, chemical etching is typically preferential, which is generally not suitable for selectively removing preform surface material.
Mechanical milling is adaptable to the normal processing environment, but can introduce mechanical stress into the glass preform, and can lead to preform structural failure (e.g., formation of cracks that can propagate through the perform).
Imperfections can also be removed quickly and cleanly by contacting the preform surface with an electrically conducting plasma region (a plasma fireball) using a isothermal plasma torch. The surface material is substantially removed by vaporization, due to the extremely high plasma temperature (&gt;9000.degree. C. at the plasma fireball center) of the isothermal plasma torch. Though the temperatures in the tail of the plasma fireball are substantially less than at the plasma center, the temperatures are generally still several thousand degrees centigrade. These tail temperatures typically are sufficiently high to cause vaporization of most refractory dielectrics, thus removing impurities from the surface of the preform as well. Such impurities include, but are not limited to, OH-type impurities, particularly water. OH-type impurities are the result of hydrogen reacting with oxygen during the deposition. The source of hydrogen comes from precursor chemicals, the starting tube or hydrogen diffusion into the preform from the heat source.
Although the above methods are sufficient for removing impurities and imperfections from the outer surface of the preform, there still remains the problem of impurities in the inner portion of the preform that are not the result of impure chemicals. Thus, what is desired is a method for making optical fiber preforms that substantially eliminates hydrogen from the deposition process to avoid OH-type impurities in the resulting optical fiber product.